A motion transmission system in a bicycle comprises a chain extending between toothed wheels associated with the axis of the pedal cranks and with the hub of the rear wheel. When at least one of the axis of the pedal cranks and the hub of the rear wheel there is more than one toothed wheel, and the motion transmission system is therefore equipped with a gearshift, a front derailleur and/or a rear derailleur are provided for. In the case of an electronically servo-assisted gearshift, each derailleur comprises a chain guide element, also known as cage, mobile to move the chain among the toothed wheels in order to change the gear ratio, and an electromechanical actuator to move the chain guide element. The actuator in turn typically comprises a motor, typically an electric motor, coupled with the chain guide element through a linkage such as an articulated parallelogram, a rack system or a worm screw system, as well as a sensor of the position, speed and/or acceleration of the rotor or of any other moving part downstream of the rotor, down to the chain guide element itself. It is worthwhile noting that slightly different nomenclatures to that used in this context are also in use.
Control electronics changes the gear ratio automatically, for example based on one or more detected variables, such as the travel speed, the cadence of rotation of the pedal cranks, the torque applied to the pedal cranks, the slope of the travel terrain, the heart rate of the cyclist and similar, and/or based on commands manually input by the cyclist through suitable command members, for example levers and/or buttons.
In order to control the actuator, instead of assuming that the toothed wheels are equally axially spaced and therefore moving the chain guide element always by the same amount, the control electronics uses a table of values containing, for each toothed wheel, the value that a variable of the derailleur must take up to position the chain in engagement with the toothed wheel. Such a value can be a differential value with respect to the adjacent toothed wheel, or it can be an absolute value with respect to a reference, for example with respect to a reference toothed wheel or to an end of stroke condition or to a condition of lack of excitation of the motor.
From the point of view of magnitude, the command value of the actuator can for example be the distance traveled by a mobile point taken as a reference on the derailleur, the number of steps or revolutions that the motor should be made to perform, a length of excitation time of the motor, the value of a supply voltage of a motor having an excursion proportional to the voltage, furthermore it can be the value emitted by the sensor associated with the motor, a numerical value stored in a register and representative of one of the aforementioned quantities, etc.
The values of said table are nominal values, set in the factory, which take the number of toothed wheels in the gearshift group (front or rear) and the respective thicknesses and pitches into account. Typically, such nominal values provide that, in the absence of the driving signal of the actuator, namely with command value at zero, the chain is in engagement with the toothed wheel having the smallest diameter, although as can be seen from the aforementioned examples, this condition is not necessary.
In one known electronically servo-assisted gearshift, a setting operating mode, an adjustment operating mode, and a normal ride mode are implemented. In the setting mode, the chain is brought into alignment with a single preselected toothed wheel, preferably the one having the smallest diameter, and a biunique correspondence is set between the physical position of the actuator and the logic value associated with the gear ratio relative to the predetermined toothed wheel, preferably zeroing a counter to which content the nominal values of the table are related. In the adjustment mode, the chain is brought in engagement and alignment with a predetermined toothed wheel, and an adjustment variable (“offset”) of the logic value associated with the gear ratio relative to the predetermined toothed wheel is set. During the normal ride mode, the actuator is moved into physical positions determined by the logic values associated with the toothed wheels as adjusted by the adjustment variables. In this way, misalignments between the chain and one or more toothed wheels are compensated, caused for example by impacts or collisions or by small differences between the size of a replaced toothed wheel and the replacement one.